Sizes of Ferroelectricity Appearance and Disappearence in Nanosized Hafnia-Zirconia:Landau-type Theory

TL;DR

This paper analytically investigates the critical sizes for the appearance and disappearance of ferroelectricity in nanosized hafnia-zirconia thin films, considering strain effects and phase stability.

Contribution

It introduces analytical expressions for critical sizes in hafnia-zirconia, incorporating strain effects and phase diagram analysis, advancing understanding of nanoscale ferroelectricity.

Findings

Critical thickness for ferroelectricity disappearance depends on depolarization and correlation effects.

Critical size for ferroelectricity appearance depends on mismatch strain and strain relaxation mechanisms.

Analytical formulas can be applied to HfxZr1-xO2 solid solutions with known parameters.

Abstract

Nanosized hafnia-zirconia HfxZr1-xO2 in the form of thin films, multilayers and heterostructures are indispensable silicon-compatible ferroelectric materials for advanced electronic memories and logic devices. The distinctive feature of nanoscale hafnia-zirconia are the critical sizes of ferroelectricity appearance, whereas the critical sizes of ferroelectricity disappearance exist in other ferroelectrics. Using the Landau-Ginzburg-Devonshire free energy functional with higher powers, trilinear and biquadratic couplings of polar, nonpolar and antipolar order parameters, we calculated analytically the strain-dependent critical sizes of the ferroelectricity appearance and disappearance, analyzed how the size effect and mismatch strains influence the phase diagrams and polarization switching barrier in epitaxial HfO2 thin films and nano-islands with the out-of-plane spontaneous polarization. We have shown that the critical thickness/height of out-of-plane spontaneous polarization disappearance is determined by the size dependence of the depolarization field and correlation effects. The critical thickness/height of the ferroelectricity appearance is determined by the size dependence of the effective mismatch strain considering possible appearance of misfit dislocations and lateral relaxion of strains. Derived analytical expressions can be generalized for HfxZr1-xO2 solid solutions, providing that corresponding parameters of the free energy are known from the first principles calculations.